1. Introduction
The renaissance of artificial intelligence (AI) in the past few years roots in the advancement of deep neural networks (DNNs). Recently, DNNs have become an essential element and a core technique for existing and emerging AI research, as DNNs have achieved stateoftheart performance and demonstrated fundamental breakthroughs in many machine learning tasks that were once believed to be challenging
(LeCun et al., 2015). Examples include computer vision, image classification, machine translation, and speech processing, to name a few.
Despite the success of DNNs, recent studies have identified that DNNs can be vulnerable to adversarial examples  a slightly modified image can be easily generated and fool a welltrained image classifier based on DNNs with high confidence
(Szegedy et al., 2013; Goodfellow et al., 2014). Consequently, the inherent weakness of lacking robustness to adversarial examples for DNNs brings out security concerns, especially for missioncritical applications which require strong reliability, including traffic sign identification for autonomous driving and malware prevention (Evtimov et al., 2017; Hu and Tan, 2017b, a), among others.Preliminary studies on the robustness of DNNs focused on an “openbox” (whitebox) setting  they assume model transparency that allows full control and access to a targeted DNN for sensitivity analysis. By granting the ability of performing back propagation, a technique that enables gradient computation of the output with respect to the input of the targeted DNN, many standard algorithms such as gradient descent can be used to attack the DNN. In image classification, back propagation specifies the effect of changing pixel values on the confidence scores for image label prediction. Unfortunately, most real world systems do not release their internal configurations (including network structure and weights), so openbox attacks cannot be used in practice.
Throughout this paper, we consider a practical “blackbox” attack setting where one can access the input and output of a DNN but not the internal configurations. In particular, we focus on the use case where a targeted DNN is an image classifier trained by a convolutional neural network (CNN), which takes an image as an input and produces a confidence score for each class as an output. Due to application popularity and security implications, image classification based on CNNs is currently a major focus and a critical use case for studying the robustness of DNNs.
We consider two types of blackbox attacks in this paper. Given a benign example with correct labeling, an untargeted attack refers to crafting an adversarial example leading to misclassification, whereas a targeted attack refers to modifying the example in order to be classified as a desired class. The effectiveness of our proposed blackbox attack (which we call ZOO) is illustrated in Figure 1. The crafted adversarial examples from our attack not only successfully mislead the targeted DNN but also deceive human perception as the injected adversarial noise is barely noticeable. In an attacker’s foothold, an adversarial example should be made as indistinguishable from the original example as possible in order to deceive a targeted DNN (and sometimes human perception as well). However, the best metric for evaluating the similarity between a benign example and a corresponding adversarial example is still an open question and may vary in different contexts.
In what follows, we summarize recent works on generating and defending adversarial examples for DNNs, and specify the “blackbox” setting of training substitute models for adversarial attacks.
1.1. Adversarial attacks and transferability
We summarize four principal openbox methods developed for attacking image classification trained on DNNs as follows.
Fast gradient sign method (FGSM) (Goodfellow
et al., 2014): Originated from an constraint on the maximal distortion, FGSM uses the sign of the gradient from the back propagation on a targeted DNN to generate admissible adversarial examples. FGSM has become a popular baseline algorithm for improved adversarial example generation (Kurakin
et al., 2016b, a), and it can be viewed as an attack framework based on firstorder projected gradient descent (Madry et al., 2017).
Jacobianbased Saliency Map Attack (JSMA) (Papernot et al., 2016a): By constructing a Jacobianbased saliency map for characterizing the inputoutput relation of a targeted DNN,
JSMA can be viewed as a greedy attack algorithm that iteratively modifies the most significant pixel based on the saliency map for crafting adversarial examples. Each iteration,
JSMA recomputes the saliency map and uses the derivative of the DNN with respect to the input image as an indicator of modification for adversarial attacks.
In addition to image classification, JSMA has been applied to other machine learning tasks such as malware classification (Grosse et al., 2016)
, and other DNN architectures such as recurrent neural networks (RNNs)
(Papernot et al., 2016b).DeepFool (MoosaviDezfooli et al., 2016a):
Inspired from linear classification models and the fact that the corresponding separating hyperplanes indicate the decision boundaries of each class, DeepFool is an untargeted attack algorithm that aims to find the least distortion (in the sense of Euclidean distance) leading to misclassification, by projecting an image to the closest separating hyperplane. In particular, an approximate attack algorithm is proposed for DNNs in order to tackle the inherit nonlinearity for classification
(Szegedy et al., 2013).Carlini & Wagner (C&W) Attack (Carlini and Wagner, 2017b): The adversarial attack proposed by Carlini and Wagner is by far one of the strongest attacks. They formulate targeted adversarial attacks as an optimization problem, take advantage of the internal configurations of a targeted DNN for attack guidance, and use the
norm (i.e., Euclidean distance) to quantify the difference between the adversarial and the original examples. In particular, the representation in the logit layer (the layer prior to the final fully connected layer as illustrated in Figure
2) is used as an indicator of attack effectiveness. Consequently, the C&W attack can be viewed as a gradientdescent based targeted adversarial attack driven by the representation of the logit layer of a targeted DNN and the distortion. The formulation of the C&W attack will be discussed in detail in Section 3. Furthermore, Carlini and Wagner also showed that their attack can successfully bypass 10 different detections methods designed for detecting adversarial examples (Carlini and Wagner, 2017a).Transferability: In the context of adversarial attacks, transferability
means that the adversarial examples generated from one model are also very likely to be misclassified by another model. In particular, the aforementioned adversarial attacks have demonstrated that their adversarial examples are highly transferable from one DNN at hand to the targeted DNN. One possible explanation of inherent attack transferability for DNNs lies in the findings that DNNs commonly have overwhelming generalization power and local linearity for feature extraction
(Szegedy et al., 2013). Notably, the transferability of adversarial attacks brings about security concerns for machine learning applications based on DNNs, as malicious examples may be easily crafted even when the exact parameters of a targeted DNN are absent. More interestingly, the authors in (MoosaviDezfooli et al., 2016b)have shown that a carefully crafted universal perturbation to a set of natural images can lead to misclassification of all considered images with high probability, suggesting the possibility of attack transferability from one image to another. Further analysis and justification of a universal perturbation is given in
(MoosaviDezfooli et al., 2017).1.2. Blackbox attacks and substitute models
While the definition of an openbox (whitebox) attack to DNNs is clear and precise  having complete knowledge and allowing full access to a targeted DNN, the definition of a “blackbox” attack to DNNs may vary in terms of the capability of an attacker. In an attacker’s perspective, a blackbox attack may refer to the most challenging case where only benign images and their class labels are given, but the targeted DNN is completely unknown, and one is prohibited from querying any information from the targeted classifier for adversarial attacks. This restricted setting, which we call a “nobox” attack setting, excludes the principal adversarial attacks introduced in Section 1.1, as they all require certain knowledge and back propagation from the targeted DNN. Consequently, under this nobox setting the research focus is mainly on the attack transferability from one selftrained DNN to a targeted but completely accessprohibited DNN.
On the other hand, in many scenarios an attacker does have the privilege to query a targeted DNN in order to obtain useful information for crafting adversarial examples. For instance, a mobile app or a computer software featuring image classification (mostly likely trained by DNNs) allows an attacker to input any image at will and acquire classification results, such as the confidence scores or ranking for classification. An attacker can then leverage the acquired classification results to design more effective adversarial examples to fool the targeted classifier. In this setting, back propagation for gradient computation of the targeted DNN is still prohibited, as back propagation requires the knowledge of internal configurations of a DNN that are not available in the blackbox setting. However, the adversarial query process can be iterated multiple times until an attacker finds a satisfactory adversarial example. For instance, the authors in (Liu et al., 2016) have demonstrated a successful blackbox adversarial attack to Clarifai.com, which is a blackbox image classification system.
Due to its feasibility, the case where an attacker can have free access to the input and output of a targeted DNN while still being prohibited from performing back propagation on the targeted DNN has been called a practical blackbox attack setting for DNNs (Papernot et al., 2017; Carlini and Wagner, 2017b; Papernot et al., 2016; Hu and Tan, 2017b, a; Liu et al., 2016). For the rest of this paper, we also refer a blackbox adversarial attack to this setting. For illustration, the attack settings and their limitations are summarized in Figure 2. It is worth noting that under this blackbox setting, existing attacking approaches tend to make use of the power of free query to train a substitute model (Papernot et al., 2017; Papernot et al., 2016; Hu and Tan, 2017b), which is a representative substitute of the targeted DNN. The substitute model can then be attacked using any whitebox attack techniques, and the generated adversarial images are used to attack the target DNN. The primary advantage of training a substitute model is its total transparency to an attacker, and hence essential attack procedures for DNNs, such as back propagation for gradient computation, can be implemented on the substitute model for crafting adversarial examples. Moreover, since the substitute model is representative of a targeted DNN in terms of its classification rules, adversarial attacks to a substitute model are expected to be similar to attacking the corresponding targeted DNN. In other words, adversarial examples crafted from a substitute model can be highly transferable to the targeted DNN given the ability of querying the targeted DNN at will.
1.3. Defending adversarial attacks
One common observation from the development of securityrelated research is that attack and defense often come handinhand, and one’s improvement depends on the other’s progress. Similarly, in the context of robustness of DNNs, more effective adversarial attacks are often driven by improved defenses, and vice versa.
There has been a vast amount of literature on enhancing the robustness of DNNs. Here we focus on the defense methods that have been shown to be effective in tackling (a subset of) the adversarial attacks introduced in Section 1.1 while maintaining similar classification performance for the benign examples.
Based on the defense techniques, we categorize the defense methods proposed for enhancing the robustness of DNNs to adversarial examples as follows.
Detectionbased defense: Detectionbased approaches aim to differentiate an adversarial example from a set of benign examples using statistical tests or outofsample analysis.
Interested readers can refer to recent works in (Huang
et al., 2016; Metzen et al., 2017; Feinman
et al., 2017; Grosse et al., 2017; Xu et al., 2017a, b) and references therein for details. In particular, feature squeezing is shown to be effective in detecting adversarial examples by projecting an image to a confined subspace (e.g., reducing color depth of a pixel) to alleviate the exploits from adversarial attacks (Xu et al., 2017a, b).
The success of detectionbased approaches heavily relies on the assumption that the distributions of adversarial and benign examples are fundamentally distinct. However, Carlini and Wagner recently demonstrated that their attack (C&W attack) can bypass 10 different detection algorithms designed for detecting adversarial examples (Carlini and
Wagner, 2017a), which challenges the fundamental assumption of detectionbased approaches as the results
suggest that the distributions of their adversarial examples and the benign examples
are nearly indistinguishable.
Gradient and representation masking: As the use of gradients via back propagation on DNNs has been shown to be crucial to crafting adversarial examples, one natural defense mechanism is to hide the gradient information while training a DNN, known as gradient masking
. A typical example of gradient masking is the defense distillation proposed in
(Papernot et al., 2016c). The authors proposed to retrain a DNN using distillation (Hinton et al., 2015) based on the original confidence scores for classification (also known as soft labels) and introduced the concept of “temperature” in the softmax step for gradient masking. An extended version has been proposed to enhance its defense performance by incorporating modelagnostic uncertainty into retraining (Papernot and McDaniel, 2017). Although the C&W attack has shown to be able to break defensive distillation (Carlini and Wagner, 2017b), it is still considered as a baseline model for defending adversarial attacks. Another defense technique, which we call representation masking, is inspired by the finding that in the C&W attack the logit layer representation in DNNs is useful for adversarial attacks. As a result, representation masking aims to replace the internal representations in DNNs (usually the last few layers) with robust representations to alleviate adversarial attacks. For example, the authors in (Bradshaw et al., 2017) proposed to integrate DNNs with Gaussian processes and RBF kernels for enhanced robustness.Adversarial training: The rationale behind adversarial training is that DNNs are expected to be less sensitive to perturbations (either adversarial or random) to the examples if these adversarial examples are jointly used to stabilize training, known as the data augmentation method. Different data augmentation methods have been proposed to improve the robustness of DNNs. Interested readers can refer to the recent works in (Jin et al., 2015; Zheng et al., 2016; Zantedeschi et al., 2017; Tramèr et al., 2017; Madry et al., 2017) and the references therein. Notably, the defense model proposed in (Madry et al., 2017) showed promising results against adversarial attacks, including the FGSM and the C&W attack. The authors formulated defense in DNNs as a robust optimization problem, where the robustness is improved by iterative adversarial data augmentation and retraining. The results suggest that a DNN can be made robust at the price of increased network capacity (i.e., more model parameters), in order to stabilize training and alleviate the effect of adversarial examples.
1.4. Blackbox attack using zeroth order optimization: benefits and challenges
Zeroth order methods are derivativefree optimization methods, where only the zeroth order oracle (the objective function value at any ) is needed during optimization process. By evaluating the objective function values at two very close points and with a small
, a proper gradient along the direction vector
can be estimated. Then, classical optimization algorithms like gradient descent or coordinate descent can be applied using the estimated gradients. The convergence of these zeroth order methods has been proved in optimization literature (Nesterov et al., 2011; Ghadimi and Lan, 2013; Lian et al., 2016), and under mild assumptions (smoothness and Lipschitzian gradient) they can converge to a stationary point with an extra error term which is related to gradient estimation and vanishes when .Our proposed blackbox attack to DNNs in Section 3 is cast as an optimization problem. It exploits the techniques from zeroth order optimization and therefore spares the need of training a substitute model for deploying adversarial attacks. Although it is intuitive to use zeroth order methods to attack a blackbox DNN model, applying it naively can be impractical for large models. For example, the Inceptionv3 network (Szegedy et al., 2016) takes input images with a size of , and thus has variables (pixels) to optimize. To evaluate the estimated gradient of each pixel, we need to evaluate the model twice. To just obtain the estimated gradients of all pixels, evaluations are needed. For a model as large as Inceptionv3, each evaluation can take tens of milliseconds on a single GPU, thus it is very expensive to even evaluate all gradients once. For targeted attacks, sometimes we need to run an iterative gradient descent with hundreds of iterations to generate an adversarial image, and it can be forbiddingly expensive to use zeroth order method in this case.
In the scenario of attacking blackbox DNNs, especially when the image size is large (the variable to be optimized has a large number of coordinates), a single step of gradient descent can be very slow and inefficient, because it requires estimating the gradients of all coordinates to make a single update. Instead, we propose to use a coordinate descent method to iteratively optimize each coordinate (or a small batch of coordinates). By doing so, we can accelerate the attack process by efficiently updating coordinates after only a few gradient evaluations. This idea is similar to DNN training for large datasets, where we usually apply stochastic gradient descent using only a small subset of training examples for efficient updates, instead of computing the full gradient using all examples to make a single update. Using coordinate descent, we update coordinates by small batches, instead of updating all coordinates in a single update as in gradient descent. Moreover, this allows us to further improve the efficiency of our algorithm by using carefully designed sampling strategy to optimize important pixels first. We will discuss the detailed algorithm in Section
3.1.5. Contributions
We refer to the proposed attack method as blackbox attacks using zeroth order optimization, or ZOO for short. Below we summarize our main contributions:
We show that a coordinate descent based method using only the zeroth order oracle (without gradient information) can effectively attack blackbox DNNs. Comparing to the substitute model based blackbox attack (Papernot et al., 2017), our method significantly increases the success rate for adversarial attacks, and attains comparable performance to the stateoftheart whitebox attack (C&W attack).
In order to speed up the computational time and reduce number of queries for our blackbox attacks to largescale DNNs,
we propose several techniques including attackspace dimension reduction, hierarchical attacks and importance sampling.
In addition to datasets of small image size (MNIST and CIFAR10), we demonstrate the applicability of our blackbox attack model to a large DNN  the Inceptionv3 model trained on ImageNet. Our attack is capable of crafting a successful adversarial image within a reasonable time, whereas the substitute model based blackbox attack in
(Papernot et al., 2017) only shows success in small networks trained on MNIST and is hardly scalable to the case of ImageNet.
2. Related Work
The study of adversarial attacks roots in the need for understanding the robustness of stateoftheart machine learning models (Barreno et al., 2006; Barreno et al., 2010)
. For instance, Biggio et el. proposed an effective attack to sabotaging the performance (test accuracy) of support vector machines (SVMs) by intelligently injecting adversarial examples to the training dataset
(Biggio et al., 2012). Gradient based evasion attacks to SVMs and multilayer perceptrons are discussed in
(Biggio et al., 2013). Given the popularity and success of classification models trained by DNNs, in recent years there has been a surge in interest toward understanding the robustness of DNNs. A comprehensive overview of adversarial attacks and defenses for DNNs is given in Section 1.Here we focus on related work on the blackbox adversarial attack setting for DNNs. As illustrated in Figure 2, the blackbox setting allows free query from a targeted DNN but prohibits any access to internal configurations (e.g., back propagation), which fits well to the scenario of publicly accessible machine learning services (e.g., mobile apps, image classification service providers, and computer vision packages). Under this blackbox setting, the methodology of current attacks concentrates on training a substitute model and using it as a surrogate for adversarial attacks (Papernot et al., 2017; Carlini and Wagner, 2017b; Papernot et al., 2016; Hu and Tan, 2017b, a; Liu et al., 2016). In other words, a blackbox attack is made possible by deploying a whitebox attack to the substitute model. Therefore, the effectiveness of such blackbox adversarial attacks heavily depends on the attack transferability from the substitute model to the target model. Different from the existing approaches, we propose a blackbox attack via zeroth order optimization techniques. More importantly, the proposed attack spares the need for training substitute models by enabling a “pseudo back propagation” on the target model. Consequently, our attack can be viewed “as if it was” a whitebox attack to the target model, and its advantage over current blackbox methods can be explained by the fact that it avoids any potential loss in transferability from a substitute model. The performance comparison between the existing methods and our proposed blackbox attack will be discussed in Section 4.
In principle, our blackbox attack technique based on zeroth order optimization is a general framework that can be applied to any whitebox attacks requiring back propagation on the targeted DNN. We note that all the effective adversarial attacks discussed in Section 1.1 have such a requirement, as back propagation on a targeted DNN provides invaluable information for an attacker. Analogously, one can view an attacker as an optimizer and an adversarial attack as an objective function to be optimized. Back propagation provides firstorder evaluation (i.e., gradients) of the objective function for the optimizer for efficient optimization. For the purpose of demonstration, in this paper we proposed a blackbox attack based on the formulation of the C&W attack, since the (whitebox) C&W attack has significantly outperformed the other attacks discussed in Section 1.1 in terms of the quality (distortion) of the crafted adversarial examples and attack transferability (Carlini and Wagner, 2017b). Experimental results in Section 4 show that our blackbox version is as effective as the original C&W attack but at the cost of longer processing time for implementing pseudo back propagation. We also compare our blackbox attack with the blackbox attack via substitute models in (Papernot et al., 2017), which trains a substitute model as an attack surrogate based on Jacobian saliency map (Papernot et al., 2016a). Experimental results in Section 4 show that our attack significantly outperforms (Papernot et al., 2017), which can be explained by the fact that our attack inherits the effectiveness of the stateoftheart C&W attack, and also by the fact that zeroth order optimization allows direct adversarial attacks to a targeted DNN and hence our blackbox attack does not suffer from any loss in attack transferability from a substitute model.
3. ZOO: A Blackbox Attack without Training Substitute Models
3.1. Notation for deep neural networks
As illustrated in Figure 2, since we consider the blackbox attack setting where free query from a targeted DNN is allowed while accessing to internal states (e.g., performing back propagation) is prohibited, it suffices to use the notation to denote a targeted DNN. Specifically, the DNN takes an image (a dimensional column vector) as an input and outputs a vector of confidence scores for each class, where is the number of classes. The th entry specifies the probability of classifying as class , and .
In principle, our proposed blackbox attack via zeroth order optimization (ZOO) can be applied to nonDNN classifiers admitting the same input and output relationship. However, since DNNs achieved stateoftheart classification accuracy in many image tasks, in this paper we focus on the capability of our blackbox adversarial attack to DNNs.
3.2. Formulation of C&W attack
Our blackbox attack is inspired by the formulation of the C&W attack (Carlini and Wagner, 2017b), which is one of the strongest whitebox adversarial attacks to DNNs at the time of our work. Given an image , let denote the adversarial example of with a targeted class label toward misclassification. The C&W attack finds the adversarial example by solving the following optimization problem:
(1)  
where denotes the Euclidean norm ( norm) of a vector , and is a regularization parameter.
The first term in (1) is the regularization used to enforce the similarity between the adversarial example and the image in terms of the Euclidean distance, since is the adversarial image perturbation of relative to . The second term in (1
) is the loss function that reflects the level of unsuccessful adversarial attacks, and
is the target class. Carlini and Wagner compared several candidates for and suggested the following form for effective targeted attacks (Carlini and Wagner, 2017b):(2) 
where is the logit layer representation (logits) in the DNN for such that represents the predicted probability that belongs to class , and is a tuning parameter for attack transferability. Carlini and Wagner set for attacking a targeted DNN, and suggested large when performing transfer attacks. The rationale behind the use of the loss function in (2) can be explained by the softmax classification rule based on the logit layer representation; the output (confidence score) of a DNN is determined by the softmax function:
(3) 
Therefore, based on the softmax decision rule in (3), implies that the adversarial example attains the highest confidence score for class and hence the targeted attack is successful. On the other hand, implies that the targeted attack using is unsuccessful. The role of ensures a constant gap between and , which explains why setting large is effective in transfer attacks.
Finally, the box constraint implies that the adversarial example has to be generated from the valid image space. In practice, every image can satisfy this box constraint by dividing each pixel value by the maximum attainable pixel value (e.g., 255). Carlini and Wagner remove the box constraint by replacing with , where . By using this changeofvariable, the optimization problem in (1) becomes an unconstrained minimization problem with as an optimizer, and typical optimization tools for DNNs (i.e., back propagation) can be applied for solving the optimal and obtain the corresponding adversarial example .
3.3. Proposed blackbox attack via zeroth order stochastic coordinate descent
The attack formulation using (1) and (2) presumes a whitebox attack because (i): the logit layer representation in (2) is an internal state information of a DNN; and (ii) back propagation on the targeted DNN is required for solving (1). We amend our attack to the blackbox setting by proposing the following approaches: (i) modify the loss function in (1) such that it only depends on the output of a DNN and the desired class label ; and (ii) compute an approximate gradient using a finite difference method instead of actual back propagation on the targeted DNN, and solve the optimization problem via zeroth order optimization. We elucidate these two approaches below.
Loss function based on : Inspired by (2), we propose a new hingelike loss function based on the output of a DNN, which is defined as
(4) 
where and is defined as . We note that is a monotonic function such that for any , if and only if . This implies that means attains the highest confidence score for class
. We find that the log operator is essential to our blackbox attack since very often a welltrained DNN yields a skewed probability distribution on its output
such that the confidence score of one class significantly dominates the confidence scores of the other classes. The use of the log operator lessens the dominance effect while preserving the order of confidence scores due to monotonicity. Similar to (2), in (4) ensures a constant gap between and .For untargeted attacks, an adversarial attack is successful when is classified as any class other than the original class label . A similar loss function can be used (we drop the variable for untargeted attacks):
(5) 
where is the original class label for , and represents the most probable predicted class other than .
Zeroth order optimization on the loss function:
We discuss our optimization techniques for any general function used for attacks (the regularization term in (1) can be absorbed as a part of ).
We use the symmetric difference quotient (Lax and Terrell, 2014) to estimate the gradient (defined as ):
(6) 
where is a small constant (we set in all our experiments) and is a standard basis vector with only the th component as 1. The estimation error (not including the error introduced by limited numerical precision) is in the order of . Although numerical accuracy is a concern, accurately estimating the gradient is usually not necessary for successful adversarial attacks. One example is FGSM, which only requires the sign (rather than the exact value) of the gradient to find adversarial examples. Therefore, even if our zeroth order estimations may not be very accurate, they suffice to achieve very high success rates, as we will show in our experiments.
For any , we need to evaluate the objective function times to estimate gradients of all coordinates. Interestingly, with just one more objective function evaluation, we can also obtain the coordinatewise Hessian estimate (defined as ):
(7) 
Remarkably, since only needs to be evaluated once for all coordinates, we can obtain the Hessian estimates without additional function evaluations.
It is worth noting that stochastic gradient descent and batch gradient descent are two most commonly used algorithms
for training DNNs, and the C&W attack (Carlini and
Wagner, 2017b) also used gradient descent to attack a DNN in the whitebox setting.
Unfortunately, in the blackbox setting, the network structure is unknown and the gradient computation via back propagation is prohibited. To tackle this problem,
a naive solution is applying (6) to estimate gradient, which requires objective function evaluations. However, this naive solution is too expensive in practice. Even for an input image size of , one full gradient descent step requires evaluations, and typically hundreds of iterations may be needed until convergence.
To resolve this issue, we propose the following coordinatewise update, which only requires function evaluations for each step.
Stochastic coordinate descent: Coordinate descent methods have been extensively studied in optimization literature (Bertsekas, Bertsekas). At each iteration, one variable (coordinate) is chosen randomly and is updated by approximately minimizing the objective function along that coordinate (see Algorithm 1 for details). The most challenging part in Algorithm 1 is to compute the best coordinate update in step 3. After estimating the gradient and Hessian for , we can use any first or second order method to approximately find the best . In firstorder methods, we found that ADAM (Kingma and Ba, 2014)’s update rule significantly outperforms vanilla gradient descent update and other variants in our experiments, so we propose to use a zerothorder coordinate ADAM, as described in Algorithm 2. We also use Newton’s method with both estimated gradient and Hessian to update the chosen coordinate, as proposed in Algorithm 3. Note that when Hessian is negative (indicating the objective function is concave along direction ), we simply update by its gradient. We will show the comparison of these two methods in Section 4. Experimental results suggest coordinatewise ADAM is faster than Newton’s method.
Note that for algorithmic illustration we only update one coordinate for each iteration. In practice, to achieve the best efficiency of GPU, we usually evaluate the objective in batches, and thus a batch of and can be estimated. In our implementation we estimate pixels’ gradients and Hessians per iteration, and then update coordinates in a single iteration.
3.4. Attackspace dimension reduction
We first define and to be the adversarial noise added to the original image . Our optimization procedure starts with . For networks with a large input size , optimizing over (we call it attackspace) using zeroth order methods can be quite slow because we need to estimate a large number of gradients.
Instead of directly optimizing , we introduce a dimension reduction transformation where , , and . The transformation can be linear or nonlinear. Then, we use to replace in (1):
(8)  
The use of effectively reduces the dimension of attackspace from to . Note that we do not alter the dimension of an input image but only reduce the permissible dimension of the adversarial noise. A convenient transformation is to define to be the upscaling operator that resizes as a size
image, such as the bilinear interpolation method
^{1}^{1}1See the details at https://en.wikipedia.org/wiki/Bilinear_interpolation. For example, in the Inceptionv3 network can be a small adversarial noise image with dimension , while the original image dimension is . Other transformations like DCT (discrete cosine transformation) can also be used. We will show the effectiveness of this method in Section 4.3.5. Hierarchical attack
When applying attackspace dimension reduction with a small , although the attackspace is efficient to optimize using zeroth order methods, a valid attack might not be found due to the limited search space. Conversely, if a large is used, a valid attack can be found in that space, but the optimization process may take a long time. Thus, for large images and difficult attacks, we propose to use a hierarchical attack scheme, where we use a series of transformations with dimensions to gradually increase during the optimization process. In other words, at a specific iteration (according to the dimension increasing schedule) we set to increase the dimension of from to ( denotes the inverse transformation of ).
For example, when using image scaling as the dimension reduction technique, upscales from to , and upscales from to . We start with variables to optimize with and use as the transformation, then after a certain number of iterations (when the decrease in the loss function is inapparent, indicating the need of a larger attackspace), we upscale from to , and use for the following iterations.
3.6. Optimize the important pixels first
One benefit of using coordinate descent is that we can choose which coordinates to update. Since estimating gradient and Hessian for each pixel is expensive in the blackbox setting, we propose to selectively update pixels by using importance sampling. For example, pixels in the corners or at the edges of an image are usually less important, whereas pixels near the main object can be crucial for a successful attack. Therefore, in the attack process we sample more pixels close to the main object indicated by the adversarial noise.
We propose to divide the image into regions, and assign sampling probabilities according to how large the pixel values change in that region. We run a max pooling of the absolute pixel value changes in each region, upsample to the desired dimension, and then normalize all values such that they sum up to 1. Every few iterations, we update these sampling probabilities according to the recent changes. In Figure 3, we show a practical example of pixel changes and how importance sampling probabilities are generated when attacking the bagel image in Figure 1 (a).
When the attackspace is small (for example, ), we do not use importance sampling to sufficiently search the attackspace. When we gradually increase the dimension of attackspace using hierarchical attack, incorporating importance sampling becomes crucial as the attackspace is increasingly larger. We will show the effectiveness of importance sampling in Section 4.
4. Performance Evaluation
4.1. Setup
We compare our attack (ZOO) with Carlini & Wagner’s (C&W) whitebox attack (Carlini and Wagner, 2017b) and the substitute model based blackbox attack (Papernot et al., 2017). We would like to show that our blackbox attack can achieve similar success rate and distortion as the whitebox C&W attack, and can significantly outperform the substitute model based blackbox attack, while maintaining a reasonable attack time.
MNIST  
Untargeted  Targeted  
Success Rate  Avg.  Avg. Time (per attack)  Success Rate  Avg.  Avg. Time (per attack)  
Whitebox (C&W)  100 %  1.48066  0.48 min  100 %  2.00661  0.53 min 
Blackbox (Substitute Model + FGSM)  40.6 %    0.002 sec (+ 6.16 min)  7.48 %    0.002 sec (+ 6.16 min) 
Blackbox (Substitute Model + C&W)  33.3 %  3.6111  0.76 min (+ 6.16 min)  26.74 %  5.272  0.80 min (+ 6.16 min) 
Proposed blackbox (ZOOADAM)  100 %  1.49550  1.38 min  98.9 %  1.987068  1.62 min 
Proposed blackbox (ZOONewton)  100 %  1.51502  2.75 min  98.9 %  2.057264  2.06 min 
CIFAR10  
Untargeted  Targeted  
Success Rate  Avg.  Avg. Time (per attack)  Success Rate  Avg.  Avg. Time (per attack)  
Whitebox (C&W)  100 %  0.17980  0.20 min  100 %  0.37974  0.16 min 
Blackbox (Substitute Model + FGSM)  76.1 %    0.005 sec (+ 7.81 min)  11.48 %    0.005 sec (+ 7.81 min) 
Blackbox (Substitute Model + C&W)  25.3 %  2.9708  0.47 min (+ 7.81 min)  5.3 %  5.7439  0.49 min (+ 7.81 min) 
Proposed Blackbox (ZOOADAM)  100 %  0.19973  3.43 min  96.8 %  0.39879  3.95 min 
Proposed Blackbox (ZOONewton)  100 %  0.23554  4.41 min  97.0 %  0.54226  4.40 min 
Our experimental setup is based on Carlini & Wagner’s framework^{2}^{2}2https://github.com/carlini/nn_robust_attacks with our ADAM and Newton based zeroth order optimizer included. For substitute model based attack, we use the reference implementation (with necessary modifications) in CleverHans^{3}^{3}3https://github.com/tensorflow/cleverhans/blob/master/tutorials/mnist_blackbox.py for comparison. For experiments on MNIST and CIFAR, we use a Intel Xeon E52690v4 CPU with a single NVIDIA K80 GPU; for experiments on ImageNet, we use a AMD Ryzen 1600 CPU with a single NVIDIA GTX 1080 Ti GPU. Our experimental code is publicly available^{4}^{4}4https://github.com/huanzhang12/ZOOAttack. For implementing zeroth order optimization, we use a batch size of ; i.e., we evaluate 128 gradients and update 128 coordinates per iteration. In addition, we set unless specified.
4.2. MNIST and CIFAR10
DNN Model. For MNIST and CIFAR10, we use the same DNN model as in the C&W attack ((Carlini and Wagner, 2017b), Table 1). For substitute model based attack, we use the same DNN model for both the target model and the substitute model. If the architecture of a targeted DNN is unknown, blackbox attacks based on substitute models will yield worse performance due to model mismatch.
Target images. For targeted attacks, we randomly select 100 images from MNIST and CIFAR10 test sets, and skip the original images misclassified by the target model. For each image, we apply targeted attacks to all 9 other classes, and thus there are 900 attacks in total. For untargeted attacks, we randomly select 200 images from MNIST and CIFAR10 test sets.
Parameter setting. For both ours and the C&W attack, we run a binary search up to 9 times to find the best (starting from 0.01), and terminate the optimization process early if the loss does not decrease for 100 iterations. We use the same step size and ADAM parameters for all methods. For the C&W attack, we run 1,000 iterations; for our attack, we run 3,000 iterations for MNIST and 1,000 iterations for CIFAR. Note that our algorithm updates far less variables because for each iteration we only update
pixels, whereas in the C&W attack all pixels are updated based on the full gradient in one iteration due to the whitebox setting. Also, since the image size of MNIST and CIFAR10 is small, we do not reduce the dimension of attackspace or use hierarchical attack and importance sampling. For training the substitute model, we use 150 holdout images from the test set and run 5 Jacobian augmentation epochs, and set the augmentation parameter
. We implement FGSM and the C&W attack on the substitute model for both targeted and untargeted transfer attacks to the blackbox DNN. For FGSM, the perturbation parameter , as it is shown to be effective in (Papernot et al., 2017). For the C&W attack, we use the same settings as the whitebox C&W, except for setting for attack transferability and using 2,000 iterations.Other tricks. When attacking MNIST, we found that the changeofvariable via can cause the estimated gradients to vanish due to limited numerical accuracy when pixel values are close to the boundary (0 or 1). Alternatively, we simply project the pixel values within the box constraints after each update for MNIST. But for CIFAR10, we find that using changeofvariable converges faster, as most pixels are not close to the boundary.
Results. As shown in Table 1, our proposed attack (ZOO) achieves nearly 100% success rate. Furthermore, the distortions are also close to the C&W attack, indicating our blackbox adversarial images have similar quality as the whitebox approach (Figures 4 and 5). Notably, our success rate is significantly higher than the substitute model based attacks, especially for targeted attacks, while maintaining reasonable average attack time. When transferring attacks from the substitute models to the target DNN, FGSM achieves better success rates in some experiments because it uses a relatively large and introduces much more noise than C&W attack. We also find that ADAM usually works better than the Newton’s method in terms of computation time and distortion.
4.3. Inception network with ImageNet
Attacking a large blackbox network like Inceptionv3 (Szegedy et al., 2016) can be challenging due to large attackspace and expensive model evaluation. Blackbox attacks via substitute models become impractical in this case, as a substitute model with a large enough capacity relative to InceptionV3 is needed, and a tremendous amount of costly Jacobian data augmentation is needed to train this model. On the other hand, transfer attacks may suffer from lower success rate comparing to whitebox attacks, especially for targeted attacks. Here we apply the techniques proposed in Section 3.4, 3.5 and 3.6 to efficiently overcome the optimization difficulty toward effective and efficient blackbox attacks.
Untargeted blackbox attacks to Inceptionv3.
Target images. We use 150 images from the ImageNet test set for untargeted attacks. To justify the effectiveness of using attackspace dimension reduction, we exclude small images in the test set and ensure that all the original images are at least in size. We also skip all images that are originally misclassified by Inceptionv3.
Success Rate  Avg.  

Whitebox (C&W)  100 %  0.37310 
Proposed blackbox (ZOOADAM)  88.9 %  1.19916 
Blackbox (Substitute Model)  N.A.  N.A. 
Attack techniques and parameters. We use an attackspace of only (the original input space is ) and do not use hierarchical attack. We also set a hard limit of iterations for each attack, which takes about 20 minutes per attack in our setup. In fact, during iterations, only gradients are evaluated, which is even less than the total number of pixels () of the input image. We fix in all Inceptionv3 experiments, as it is too costly to do binary search in this case. For both C&W and our attacks, we use step size 0.002.
Results. We compare the success rate and average distortion between our ZOO attack and the C&W whitebox attack in Table 2. Despite running only 1,500 iterations (within 20 minutes per image) and using a small attackspace (), our blackbox attack achieves about 90% success rate. The average distortion is about 3 times larger than the whitebox attack, but our adversarial images are still visually indistinguishable (Figures 1). The success rate and distortion can be further improved if we run more iterations.
Targeted blackbox attacks to Inceptionv3.
For Inceptionv3, a targeted attack is much more difficult as there are 1000 classes, and a successful attack means one can manipulate the predicted probability of any specified class. However, we report that using our advanced attack techniques, iterations (each with pixel updates) are sufficient for a hard targeted attack.
Target image. We select an image (Figure 1 (a)) for which our untargeted attack failed, i.e., we cannot even find an untargeted attack in the attackspace, indicating that this image is hard to attack. Inceptionv3 classifies it as a “bagel” with 97.0% confidence, and other top5 predictions include “guillotine”, “pretzel”, “Granny Smith” and “dough” with 1.15%, 0.07%, 0.06% and 0.01% confidence. We deliberately make the attack even harder by choosing the target class as “grand piano”, with original confidence of only 0.0006%.
Attack techniques. We use attackspace dimension reduction as well as hierarchical attack. We start from an attackspace of , and increase it to and at iteration 2,000 and 10,000, respectively. We run the zeroth order ADAM solver (Algorithm 2) with a total of 20,000 iterations, taking about 260 minutes in our setup. Also, when the attack space is greater than , we incorporate importance sampling, and keep updating the sampling probability after each iteration.
Reset ADAM states. We report an additional trick to reduce the final distortion  reset the ADAM solver’s states when a first valid attack is found during the optimization process. The reason is as follows. The total loss consists of two parts: and . measures the difference between the original class probability and targeted class probability as defined in (4). When , , and a valid adversarial example is found. is the distortion. During the optimization process, we observe that before reaches 0, is likely to increase, i.e., adding more distortion and getting closer to the target class. After reaches 0 it cannot go below 0 because it is a hingelike loss, and at this point the optimizer should try to reduce as much as possible while keeping only slightly larger than . However, when we run coordinatewise ADAM, we found that even after reaches 0, the optimizer still tries to reduce and to increase , and will not be decreased efficiently. We believe the reason is that the historical gradient statistics stored in ADAM states are quite stale due to the large number of coordinates. Therefore, we simply reset the ADAM states after reaches 0 for the first time in order to make the solver focus on decreasing afterwards.
Blackbox (ZOOADAM)  Success?  First Valid  Final  Final Loss 

All techniques  Yes  15,227  3.425  11.735 
No Hierarchical Attack  No      62.439 
No importance sampling  Yes  17,403  3.63486  13.216 
No ADAM state reset  Yes  15,227  3.47935  12.111 
Results. Figure 6 shows how the loss decreases versus iterations, with all techniques discussed above applied in red; other curves show the optimization process without a certain technique but all others included. The black curve decreases very slowly, suggesting hierarchical attack is extremely important in accelerating our attack, otherwise the large attackspace makes zeroth order methods infeasible. Importance sampling also makes a difference especially after iteration 10,000 – when the attackspace is increased to ; it helps us to find the first valid attack over 2,000 iterations earlier, thus leaving more time for reducing the distortion. The benefit of reseting ADAM states is clearly shown in Table 3, where the final distortion and loss increase noticeably if we do not reset the states.
The proposed ZOO attack succeeds in decreasing the probability of the original class by over 160x (from 97% to about 0.6%) while increasing the probability of the target class by over 1000x (from 0.0006% to over 0.6%, which is top1) to achieve a successful attack. Furthermore, as shown in Figures 1, the crafted adversarial noise is almost negligible and indistinguishable by human eyes.
5. Conclusion and Future Work
This paper proposed a new type of blackbox attacks named ZOO to DNNs without training any substitute model as an attack surrogate. By exploiting zeroth order optimization for deploying pseudo back propagation on a targeted blackbox DNN, experimental results show that our attack attains comparable performance to the stateoftheart whitebox attack (Carlini and Wagner’s attack). In addition, our blackbox attack significantly outperforms the substitute model based blackbox attack in terms of attack success rate and distortion, as our method does not incur any performance loss in attack transferability. Furthermore, we proposed several acceleration techniques for applying our attack to large DNNs trained on ImageNet, whereas the substitute model based blackbox attack is hardly scalable to a large DNN like Inceptionv3.
Based on the analysis and findings from the experimental results on MNIST, CIFAR10 and ImageNet, we discuss some potential research directions of our blackbox adversarial attacks as follows.
Accelerated blackbox attack: although our blackbox attack spares the need for training substitution models and attains comparable performance to the whitebox C&W attack, in the optimization process it requires more iterations than a whitebox attack due to additional computation for approximating the gradient of a DNN and for performing pseudo back propagation. In addition to the computation acceleration tricks proposed in Section 3, a datadriven approach that take these tricks into consideration while crafting an adversarial example can make our blackbox attack more efficient.
Adversarial training using our blackbox attack: adversarial training in DNNs is usually implemented in a whitebox setting  generating adversarial examples from a DNN to stabilize training and making the retrained model more robust to adversarial attacks. Our blackbox attack can serve as an independent indicator of the robustness of a DNN for adversarial training.
Blackbox attacks in different domains: in this paper we explicitly demonstrated our blackbox attack to image classifiers trained by DNNs. A natural extension is adapting our attack to other data types (e.g., speeches, time series, graphs) and different machine learning models and neural network architectures (e.g., recurrent neural networks). In addition, how to incorporate side information of a dataset (e.g., expert knowledge) and existing adversarial examples (e.g., security leaks and exploits) into our blackbox attack is worthy of further investigation.
Acknowledgment
The authors would like to warmly thank Florian Tramèr andTsuiWei Weng for their valuable feedbacks and insightful comments. ChoJui Hsieh and Huan Zhang acknowledge the support of NSF via IIS1719097.
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Appendix
Figure 7 displays some additional adversarial examples in ImageNet from our untargeted attack.
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